The Session Initiation Protocol (SIP) is one of the framework agreements for multimedia communication systems formulated by the Internet Engineering Task Force (IETF). The SIP is a text-based application-layer control protocol, which is independent of the lower-layer protocols and is designed to create, modify and terminate two-party or multi-party multimedia sessions on an IP network. The SIP technology provides a forking mechanism so that a SIP message can be received by multiple user equipments (UEs) correlated with a user ID. Forking comes in two types: parallel forking and serial forking. Parallel forking means that a proxy sends a SIP message to multiple UEs concurrently. Serial forking means the proxy sends a SIP message to each of multiple UEs one by one; if a UE returns a success response, the proxy stops sending SIP messages to other UEs; otherwise, the proxy continues sending SIP messages to other UEs until the SIP message is sent to all UEs.
The IMS is a subsystem proposed in the 3rd Generation Project Partnership (3GPP) Release 5 standard to support IP multimedia services. The IMS is based on SIP, and uses a SIP call control mechanism to create, manage and terminate various multimedia services. The IMS adopts the SIP-based forking mechanism, which is implemented by the Serving Call Session Control Function (S-CSCF) of the IMS. The S-CSCF receives registration information of the UE, and correlates the public user ID with the UE address. When a public user ID is correlated with multiple UE addresses, the SIP message sent to the public user ID is forked at the S-CSCF. The user configures whether the S-CSCF performs parallel forking or serial forking for the received messages. Namely, the user who correlates multiple UEs with the same public user ID can configure user data to decide whether the received SIP message undergoes parallel forking or serial forking.
The short message service (SMS) is maturely developed and widely applied in the traditional circuit switched (CS) domain. In order to continue developing the SMS and enable the SMS in the future IP network, the 3GPP organization sets up a topic to study how to use the SMS over IP. Currently, the 3GPP solves the SMS over IP by introducing an IP short message gateway (IP-SM-GW) as an SMS gateway between the IMS domain and the CS domain. When an IMS domain user sends a short message to a CS domain user, the short message is encapsulated in a SIP message body, and sent to the IP-SM-GW. The IP-SM-GW extracts the short message from the SIP message body, and sends the short message to the CS domain. The short message is sent by the CS domain user to the IMS domain user, and then transferred to the IP-SM-GW. The IP-SM-GW constructs a SIP message, encapsulates the short message into the SIP message body, and sends the SIP message body to the IMS domain user. The 3GPP TS 23.040 is a standard about SMS, and stipulates that a short message can carry a maximum data input of 140 bytes; if the data input is more than 140 bytes, the UE can send the short message through multiple correlated short messages, and the message receiver can assemble the multiple correlated short messages into a complete message.
When a CS domain user sends multiple message segments to an IMS domain user, because the IMS domain user owns multiple UEs, the short message undergoes forking at the S-CSCF. When the UE receives the message, the wireless network may be instable, so that some message segments fail to be received and the short message received by the UE is incomplete. Moreover, when a short message interworks with a SIP message, upon receiving multiple message segments, the IP-SM-GW converts them into multiple SIP messages directly, and sends them to the IMS terminal, without embodying the relationships between them. However, the IMS terminal is unable to assemble the SIP messages into a complete message. Namely, the IMS terminal is unable to obtain a complete short message. Further, the message segmentation in the IMS domain brings multiple SIP messages, which increases the load on the network.